This invention relates to an improved discharge ionization detector especially useful in detecting the gaseous components in a gas chromatography system.
Gas chromatograph systems separate an unknown substance into component parts so that the exact components of the unknown substance can be determined and identified Thus, once the unknown substance has been chromatographicalIy separated into components, a detector of one sort or another is necessarY to detect the various components The type of detector to which this invention relates is the discharge ionization detector, or D.I.D.
In general, a D.I.D. operates by applying a high voltage across two discharge electrodes which are located in a gas filled discharge chamber. Typically, the gas may be helium, from which gas photons become discharged at a characteristic energy level. This "glow discharge" gas is directed toward an ionization chamber, into which sample gas (containing the unknown substance component(s)) from the gas chromatograph also is directed Inside the ionization chamber are two electrodes: one a negatively charged emitter electrode, the other a positively charged collector electrode. The glow discharge gas passing from the discharge chamber into the ionization chamber mixes with and interacts with the sample gas, causing electrons to be generated in the ionization chamber. These electrons are repelled by the emitter electrode and attracted by the collector electrode, causing a current to flow between these electrodes The magnitude of this current can be used to ascertain the composition of the unknown substance, all as is well known in the art. One such D.I.D. is shown by the U.S. Pat. No. 4,789,783 to Cook.
It has now been discovered by the inventors hereof that the exact geometry of the two chambers and of the emitter/collector electrodes in the D.I.D. are surprisingly important to the efficient operation of a D.I.D. In particular, the collector and emitter electrodes can be shaped and be positioned relative to the inlet for the sample gas and to the aperture (inlet) from the discharge chamber into the ionization chamber so that improved sensitivity for the D.I.D. results. The relative size of the chambers, a smaller ionization chamber in particular, is important for best sensitivity in the detector.